Hydration thermodynamics of aliphatic alcohols

The hydration thermodynamics of five linear aliphatic alcohols in the tempe rature range 5-100 degrees C is carefully analysed using a suitably modifie d version of the theoretical approach developed by Lee. The hydration Gibbs energy change is determined by the balance of three contributions: the dir ect alcohol-water van der Waals interaction energy, the direct alcohol-wate r H-bond energy, and the excluded volume effect due to solute insertion. Th e analysis shows that the direct alcohol-water H-bond energy is fundamental in determining the negative values of the hydration Gibbs energy over the whole temperature range investigated, whereas the excluded volume effect de termines the large and negative hydration entropies. The reorganization of H-bonds in the hydration shell of aliphatic alcohols proves to be a compens ating process, not affecting the Gibbs energy change, as in the case of the hydration of nonpolar molecules. However, H-bond reorganization is the mai n molecular origin of the large and positive hydration heat capacity change , a signature of hydrophobic hydration, determining the temperature depende nce of the hydration enthalpy and entropy changes. We show that H-bond reor ganization can be reliably described by means of the modified Muller's mode l, indicating that the hydration shell is not akin to an iceberg: hydration shell H-bonds are energetically slightly stronger but more broken than tho se in bulk water. This finding allows the rationalization of the puzzling e xperimental data on the temperature dependence of the water proton NMR chem ical shift in solutions of aliphatic alcohols.